The present invention relates to upper limb prostheses and in particular to such prostheses with mechanically (usually electro-mechanically) operable pivoting wrist, elbow and/or shoulder joints.
The design of such prostheses presents various problems in relation to design flexibility and manufacturing cost, cosmetic appearance, power, stability etc. It will be appreciated that upper limb prostheses have relatively high power and strength requirements due to, inter alia, the considerable leverage forces exerted when, for example, using the prosthesis to lift objects more or less at arm""s length. Thus previously known prostheses are essentially exoskeletal in nature with a relatively substantial large diameter shell structure shaped and finished to have the appearance of an upper limb or part thereof. With this type of construction it is necessary for a substantial part of the manufacture of the prosthesis to be customised to each individual patient, and in the case of children, to different developmental stages thereof. As the customised shell is also used as the main structural load bearing component and the support for the various operating mechanisms of the upper limb this results in relatively high costs and extended manufacturing processes. In addition, where, as is usually desirable, a rotating wrist is used, then the cosmetic appearance is compromised by the visibility of the join or discontinuity between the rotating and non-rotating parts of the prosthesis. In addition the motors used require relatively complex gearing systems and often the inclusion of secondary motors in order to provide the required power, and the necessary locking of the joint under load in a desired attitude, respectively. This in turn leads to increased weight, reduced design flexibility and relatively high power consumption.
It is an object of the present invention to avoid or minimise one or more of the above disadvantages.
It has now been found that by the use of a worm gear system for mechanically operating the prosthesis, the construction of upper limb prostheses can be very considerably simplified whilst maintaining performance comparable with or better than that of conventional externally powered upper limb prostheses.
In more detail the present invention provides an articular endoskeletal prosthesis for providing a user with at least one of a mechanically operable pivoting wrist, elbow and shoulder joint, said prosthesis having at least one elongate endoskeletal tube upper limb member with a proximal end portion having a pivotal connection to a support body therefor, one of said upper limb member proximal end portion and said support body having a fixed worm gear wheel means and the other a drive motor having a drive output worm extending generally tangentially of said worm gear wheel means for engagement with the gear teeth of said worm gear wheel means so that when said drive motor is operated, in use of the prosthesis, said upper limb member moves around said worm gear wheel means so as to pivot said upper limb member about its pivotal connection.
Thus with a prosthesis of the present invention construction may be greatly simplified by using more or less plain conventional lightweight tubing of relatively small diameter (as compared with the limb diameter) and which can readily be cut to any desired length. The relatively simple and compact form of drive mechanism also contributes to simplifying construction and reducing the strength and size requirements of the tubing and drive motor and power source requirements. Thus the cosmetic personalisation of a prosthesis to a particular patient can be substantially restricted to a non-structural outer cladding for the prosthesis. The form of construction used by the invention can moreover provide improved cosmeticisation opportunities as further discussed hereinbelow.
Another significant benefit of the present invention arises from the fact that a worm gear system is inherently substantially self-locking so that when the motor is switched off and stops driving a limb member which is still under load, the limb member is held in position and no additional mechanism is required in order to retain the limb member in a given position.
In general the drive motor means and any gearbox provided therewith, have a generally cylindrical form with an axially extending worm gear so that they can conveniently be mounted inside the end of an endoskeletal tube member with the drive output worm projecting axially outwardly therefrom. The power source could also be mounted inside the endoskeletal tube member but more conveniently could have a generally annular form (with a greater or lesser angular extent e.g. two units each having an angular extent of 180xc2x0) fitted around the outside of the endoskeletal tube member.
As noted above the prostheses of the invention may have one or more of a wrist joint, an elbow joint, and a shoulder joint, mechanically operable in accordance with the present invention. It would also be possible though to use, for example, an elbow joint according to the present invention in combination with a conventional mechanically operable wrist joint. Generally it will be more convenient to have the drive motor with its drive output worm mounted down-limb of the fixed worm gear wheel means. Thus in the case of a wrist joint the drive motor would preferably be inside the hand member and the fixed gear wheel means on the distal end of the forearm member. It would also be possible though for the drive motor to be inside the forearm member with the fixed worm gear wheel means mounted on the hand member. Similarly it would generally be preferred with an elbow joint for the drive motor to be in the forearm or lower arm member and the fixed worm gear wheels to be on the upper arm member; and with a shoulder joint, for the former to be in the upper arm member and the latter on a support body fitted in the shoulder of the patient. An advantage of the alternative arrangement of having the drive motor up-limb of the joint e.g. in the upper arm for an elbow joint, is that it raises the centre of gravity of the limb proximally thereby reducing the energy consumption and power requirements of the drive motor for that joint. It will of course be appreciated that, depending on the extent of the prosthesis, the support body for any given joint may comprise an endoskeletal tube member or some form of stump adaptor.
The endoskeletal tube members may have a variety of different forms and sizes depending, inter alia, on the materials used and the requirements of the individual patient. Thus the tube may be of polygonal e.g. square or hexagonal section, or could have a rounded, e.g.oval or elliptical section. Conveniently though there is used a generally plain cylindrical section as this is generally more easily available and more readily interfaceable with the other components of the prosthesis. The diameter may vary with strength and rigidity requirements which would generally increase from a hand member to a forearm member to an upper arm member. The tube wall thickness and material and construction will also affect the tube diameter. Advantageously the tubing is of woven and/or laminated carbon fibre which combines considerable strength with lightness. In this case the tubing could generally have a diameter in the range from 10 to 50 mm, preferably 12 to 40 mm with a wall thickness of from 0.5 mm to 5 mm, preferably from 1 to 2.5 mm. Thus for example in the case of a hand member there would typically be used tubing with a diameter of from 15 mm to 20 mm and in a lower or upper arm member tubing with a diameter of from 25 mm to 35 mm. Other materials could also be used though, e.g. high strength lightweight metal alloys such as duralumin(trademark).
Various suitable motors having a relatively high power-to-weight ratio are known in the art including permanent magnet DC motors which have a substantially linear relation between torque and drive current over a reasonably wide range which facilitates control of the driving of the finger member. Particularly suitable motors are available from Minimotor SA of Switzerland, especially their motors which have a diameter of around 10 to 30 mm. A further advantage of this type of motor is the availability of a modular gearbox system coupled to the output shaft of the motor which allows different torque-output drive speed ratios to be selected simply by choosing from a range of gearboxes with different ratios. This has the advantage of facilitating the provision, in a multi-joint prosthesis, of different operating characteristics by simply using different gearboxes in different joints.
The use of cylindrical endoskeletal tube members also has the advantage of facilitating the incorporation of a rotational capabilityxe2x80x94especially in relation to wrist joints. This may be provided in generally known manner but the use of an endoskeletal form of construction has the additional advantage of allowing the use of cosmetic cladding which extends unbroken across the joint thereby avoiding the unsightly discontinuities that are present in conventional rotating wrist joints.